This invention relates in general to drive train assemblies for transferring rotational power from an engine to an axle assembly in a vehicle. In particular, this invention relates to an electronically-controlled adjustable-height bearing support bracket for the bearing support of a multiple piece drive shaft and to a vehicle which incorporates such a bearing support bracket.
In most light trucks and vans in use today, a drive train assembly is provided for transmitting rotational power from an output shaft of a transmission to an input shaft of an axle assembly so as to rotatably drive one or more wheels of the vehicle. To accomplish this, a typical light truck or van drive train assembly includes front and rear cylindrical drive shafts. A first universal joint is connected between the output shaft of the transmission and a first end of the front drive shaft. A second universal joint is connected between a second end of the front drive shaft and rear drive shaft. A third universal joint is connected between the rear drive shaft and the differential input shaft of the rear axle assembly. The universal joints provide a rotational driving connection from the output shaft of the transmission through the front and rear drive shafts to the input shaft of the differential, while accommodating angular misalignment between the rotational axis of these four shafts.
Not only must the drive train assembly accommodate a limited amount of angular misalignment between the transmission and the differential input shaft, but it must also typically accommodate a limited amount of axial movement therebetween. A small amount of such relative axial movement frequently occurs when the vehicle is operated. To address this, it is known to provide one or more slip yoke assemblies in the drive train assembly. A typical slip yoke assembly includes first and second splined members which are connected to respective components of the drive train assembly.
A first slip yoke assembly is located between the front end of the front drive shaft and the output shaft of the transmission. A second slip yoke assembly is positioned on the rear end of the front drive shaft and the second universal joint that connects the front drive shaft with the rear drive shaft.
The engine and transmission of a vehicle are typically fixed with respect to the frame of the vehicle. However, the differential on the rear axle moves with respect to the frame of the vehicle primarily due to the loading of the vehicle. In most vehicles having a front and rear drive shaft, the rear portion of the front drive shaft is supported by a bearing which is suspended from the frame of the vehicle.
When the vehicle only has a single occupant and is unloaded, the rear drive shaft typically angles vertically downward from the universal joint connecting the front and rear drive shafts. When a vehicle is fully loaded, the springs which mount the rear axle to the vehicle frame are compressed and therefore the rear drive axle will incline upwardly from the universal joint connecting the rear drive shaft to the front drive shaft.
Conventional universal joints allow two connected shafts to rotate with one another at the same average velocity. When a torque is transmitted at an angle a bending moment is produced. This bending moment is called a secondary couple. In a drive shaft, the secondary couples react on the supporting structure. In the case of a pick-up or van type vehicle with a two piece drive train, the universal cardan joints are transmitting torque, at an angle. The secondary couple produces a dynamic oscillatory load to the supporting structure at a frequency of twice the speed of revolution.
In the two piece drive shaft this vibration is mostly transmitted through the center bearing and is felt like shaking or shudder. In the two piece drive train with the three universal joints, each joint will produce an oscillating secondary couple. Minimizing the three angles (at the universal joints) will reduce the shudder at the center bearing. Another way of minimizing the shudder is by arranging the three universal joint angles in such a way that the reacting forces of the three-cardan joints cancel each other. The change of the angles can be performed by changing the position of the center bearing.
For a live rear suspension where the rear axle position relative to the body changes depending on the vehicle load, designing a two piece drive train for minimum shudder is impossible with a fixed center bearing position. If the center bearing could move to compensate for axle movement, cancellation of the reaction forces on the center bearing could be achieved at various vehicle loadings.
Launch shudder is most notable when a vehicle initially accelerates and is a function of the torque transfer between the transmission and the drive axle. When accelerating from a stopped position, the torque transfer between the engine of the vehicle and rear wheels is typically the greatest. The greater the torque transfer and the greater the angle between the front drive shaft and the rear drive shaft, the greater will be the generation of launch shudder. In many large vehicles, launch shudder is ignored since it does not materially function to damage any components of the drive train. However, in minivans, light trucks or vans which have a chassis of a light truck or pickup truck, launch shudder can be displeasing to a vehicle occupant.
To minimize launch shudder, the position of the bearing which supports the rear end of the front drive shaft is selected such that the angles between the front drive shaft and the rear drive shaft (as much as possible) approaches an ideal 180xc2x0 , that is, a straight line. However, as mentioned previously, the angles between the front drive shaft and rear drive shaft are also dependent upon the loading of the vehicle. Therefore, in most vehicles the bearing support is placed at a theorized, ideal location which is somewhere between the extremes of the suspension system which are defined when the vehicle is loaded fully or when the vehicle is at its lightest with a single vehicle occupant. Extreme departures from this ideal setting, such as when the vehicle has its seats removed for the addition of cargo and there is only one vehicle occupant, or in conditions when the vehicle is fully occupied and loaded to its maximum capacity, bring about situations which can inadvertently facilitate the generation of launch shudder upon acceleration of the vehicle. Additionally, launch shudder can also be introduced when the vehicle is already moving and thereafter undergoes an extreme acceleration.
Launch shudder can be reduced by the utilization of constant velocity universal joints. In a constant velocity universal joint the secondary couple that is produced does not oscillate with the revolution of the shaft but is constant. Constant velocity joints have been found to greatly reduce launch shudder. However, constant velocity universal joints substantially increase the cost of the drive train. In practice, constant velocity joints have been combined with conventional universal joints in a common drive shaft as a compromise solution in regards to cost. Such drive trains exhibit launch shudder although with a reduced intensity. Still the reduced level of launch shudder can be undesirable to a vehicle occupant.
It is desirable to find a method to substantially reduce or eliminate launch shudder at a lower expense than the utilization of constant velocity universal joints in the drive train.
The present invention has been developed to meet the above-noted objects. In a preferred embodiment of the present invention, provides a mechanism that a powered driver adjusts the height of a bearing support bracket. The bearing support bracket can be adjusted based upon the angle between first and second drive shafts to reduce the angle in order to minimize launch shudder. In a preferred embodiment, the driver which adjusts the height of the bearing bracket is controlled by a controller which receives data of the angles between the front and rear drive shafts and the torque condition of the drive train.
It is an object of the present invention to provide a vehicle drive train with reduced launch shudder. It is another object of the present invention to provide a drive train with reduced launch shudder accomplished by the adjustment of the bearing support bracket which supports the front drive shaft in a two-piece drive shaft. It is also an object of the present invention to provide a drive shaft arrangement which reduces launch shudder without the utilization of constant velocity universal joints. Still another object of the present invention is to provide a drive train with an adjustable-height bearing support bracket which is controlled in relationship to the cargo load and/or rear suspension height.
The above noted features and other advantages of the present invention will become more apparent to those skilled in the art as the invention is further explained in the accompanying drawings and detailed description.